This invention relates generally to the field of gasoline-based internal combustion engines and more specifically to exhaust gas re-circulation systems.
Many lean-burn internal combustion engines include an exhaust gas re-circulation (xe2x80x9cEGRxe2x80x9d) system to address issues with nitrogen oxide (xe2x80x9cNOxxe2x80x9d) and fuel economy. A portion of the exhaust gas from the combustion chamber is recirculated via an exhaust gas re-circulation valve (xe2x80x9cEGR valvexe2x80x9d) back into the intake manifold to be mixed with fresh air and fuel mixture. The amount of exhaust gas to be re-circulated may be controlled by opening or closing the EGR valve.
Direct injection engines, also called xe2x80x9cin-cylinder injection engines,xe2x80x9d inject fuel directly into the cylinders. Recently, direct injection spark ignited (xe2x80x9cDISIxe2x80x9d) engines have been proposed. A DISI engine requires the fuel to be injected at relative high pressure. Such a DISI engine also can be operated with a leaner airfuel ratio than conventional gasoline engines.
DISI engines operate in a stratified mode or a homogenous mode. When a DISI engine is in the stratified mode, the combustion chambers contain stratified layers with different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture or a slightly richer mixture, and subsequent strata contain progressively leaner mixtures. When the engine is in the homogeneous mode, a homogeneous mixture of air and fuel is injected into the combustion chamber. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
When a DISI engine operates in the stratified mode, the fuel is injected late in the compression cycle, usually during a compression stroke. Because of the late injection, a very short time is available for mixing of the air and fuel in the cylinder. Because of the short mixing time, the fuel in the rich zone passes the spark plug during ignition. Stable combustion is obtained because the rich zone air/fuel mixture near the spark plug is within the ignition limits while the overall air/fuel mixture in the cylinder is leaner than the air/fuel mixture normally used when the engine is in the homogeneous mode. When the engine is in the homogeneous mode, fuel is injected during an intake stroke of the engine. More mixing occurs in the homogenous mode then in the stratified mode. The stratified combustion mode is more fuel efficient than the homogenous mode due to pumping loss reduction. The stratified mode may be used for light to medium loads and the homogeneous mode may be used for medium to heavy loads.
Direct injection engines are commonly coupled to three-way catalytic converters to reduce CO, HC, and NOx emissions. When operating at air/fuel mixtures lean of stoichiometry, an NOx trap or an NOx catalyst is typically coupled downstream of the three-way catalytic converter to further reduce NOx emissions.
An engine with an exhaust gas re-circulation system my operate in a homogeneous mode or a stratified mode. In the homogeneous mode, the cylinders are filled with a homogeneous air/fuel mixture. In the homogeneous mode with exhaust gas re-circulation, the cylinders are filled with a homogeneous mixture of air/fuel and exhaust gas. In a stratified mode, also called the stratified exhaust gas re-circulation mode or the SEGR mode, the cylinders are filled with a stratified mixture of air/fuel and exhaust gas.
An improved stratified exhaust gas re-circulation engine includes an exhaust gas re-circulation port that can be used as an exhaust gas port or an exhaust gas re-circulation port. The re-circulation valves that control the exhaust gas re-circulation ports may be on a separate camshaft from the intake valves and the exhaust valves. The exhaust gas re-circulation ports may be helical or tangential ports. The exhaust gas and the air may be radially stratified, such that the exhaust gas is circulated around the air in the cylinders. One or more of the intake ports for each cylinder may be a helical or tangential port that circulates the air within the circling exhaust gas. The engine may be a direct injection or a port fuel injection engine with one or more exhaust manifolds.
In another embodiment of the engine, the engine uses a flow valve to control the exhaust gas re-circulation, while the re-circulation valve for the exhaust gas re-circulation port is driven by a different cam shaft as the intake and exhaust valves. The engine may use an intake flow valve(s) to control the air intake. In a five valve embodiment of the engine, the engine includes two intake valves, two exhaust valves, and a re-circulation valve.
The foregoing discussion has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention.